In a downhole mining process of resources such as coal, the mining activity damages the original stress equilibrium state in the coal rock, and results in redistribution of stress around the mining space, forming a “three-area” distribution. Mine pressure behavior phenomena such as roof caving and weighting, roof-to-floor convergence, support compression and fracturing under load, and dynamic phenomena such as water inrush, coal and gas outburst, and rock burst, etc., may occur around the stope and mining roadways, owing to surrounding rock deformation, displacement, and damage under the action of stress in the surrounding rocks.
The stress intensity in a coal and rock mass is a fundamental parameter in prevention and control of coal rock dynamic disasters, such as coal mining, support and roof design, rock burst and coal and gas outburst, etc. Testing and evaluating the stress in the coal and rock mass is a major task in mine pressure observation. Therefore, monitoring the stress in the coal and rock mass in real time provides a decision basis for solving major technical problems, such as control of mine pressure in roadways affected by mining, mining procedure design, appropriate selection and maintenance of roadway position, prediction and control of rock burst and coal and gas outburst, and safe mining of coal mass above confined aquifer, etc.
The stresses in the coal and rock mass are balanced in overall and coupled with each other before the coal and rock mass affected by the mining; under the impact of mining, local mining-induced stresses or energy is accumulated or dissipated, and the stresses at different positions on the same axis are different with each other. In the past, the stress measurement is mainly focused on individual measuring points, and synchronous measurement of stresses at multiple measuring points on an axis in the coal and rock mass cannot be achieved; hence, the synchronous variations of stresses at the points in the drilling depth direction cannot be reflected. Such measurements are far from enough for revealing the rule of stress distribution and synchronous variation in a coal and rock mass. In contrast, utilizing sensors in different depths in different drilled holes, the stress variation in different depths in a coal and rock mass can be reflected; the synchronous and real-time monitoring of stresses at multiple measuring points is closer to the actual engineering situation. Therefore, monitoring the stresses at multiple measuring points in the coal and rock mass synchronously is of great practical significance and reference value for revealing the rule of stress distribution, determining the risk of dynamic disasters, and preventing and controlling dynamic disasters of coal rocks.
Coal and rock mass stress testing is a very complex engineering. At present, many payoffs have been achieved in coal and rock mass stress testing and monitoring. Stress monitoring methods, such as flat jack method, stress-relief method, hydraulic fracturing method, electromagnetic radiation method, sound emission method, etc., have been put forth; and stress monitoring devices, such as CSIRO hollow inclusion strain gauge, UNSW hollow inclusion strain gauge, small-bore hydraulic cracking ground stress testers, telescopic mounter head of deep-hole ground stress detector, borehole deformation gauge, oil pressure pillow, etc., have been developed. However, all of these methods and devices cannot be used to realize real-time monitoring of the stresses at different depths in a drilled hole in a coal and rock mass. Hence, it is very necessary to design a device for synchronously monitoring the stresses at different depths in a drilled hole in a coal and rock mass, which is easy to install and highly adaptive.